In the paper industry, starch is used as an adhesive in coating compositions. The starch binds the pigment to the paper web and also bonds pigment particles together. Starches which are employed as adhesives in coating compositions are usually modified by one of several methods, such as enzymatic degradation, acid hydrolysis, oxidation, or dextrinization. Each is successful to some degree in producing a starch of a decreased, reduced viscosity for use in coatings.
The effect of starch modification is to alter to some extent the molecular nature of the starch and to permit the control and reduction of starch viscosity.
As the viscosity of a starch suspension is reduced, undesirable side effects also occur. Some of these side effects are discoloration, retrogradation of the converted material, high viscosity if the product is cooled too quickly or stored too long before use, and loss of adhesive strength.
Maurer U.S. Pat. No. 3,475,215 mentions the use of an acid hydrolysis catalyst in combination with hydrogen peroxide as an oxidizing agent in a process for modifying starch for paper coating compositions. If hydrogen peroxide is not used in Maurer's process, the final viscosity is extremely high, too high for use as adhesive in a paper coating composition. Moreover, failure to use hydrogen peroxide also results in excessive retrogradation.
Our co-pending application Voigt and Bovier U.S. patent application Ser. No. 475,619, filed June 3, 1974, U.S. Pat. No. 3,973,985, entitled METHOD OF DECREASING STARCH VISCOSITY DURING PASTING discloses a method of reducing starch viscosity by oxidizing a lightly oxidized starch with ammonium persulfate.